Fix the macOS build.

Author: kazutakahirata

llvm/lib/Support/RWMutex.cpp

@@ -13,6 +13,7 @@
 #include "llvm/Support/RWMutex.h"
 #include "llvm/Config/config.h"
 #include "llvm/Config/llvm-config.h" // for LLVM_ENABLE_THREADS
+#include "llvm/Support/Allocator.h"
 
 #if defined(LLVM_USE_RW_MUTEX_IMPL)
 using namespace llvm;